Method for monitoring of medical treatment using pulse-echo ultrasound

ABSTRACT

A method for ultrasound imaging of anatomical tissue. A first signal is received from a first imaging ultrasound wave which has been reflected from a location in the anatomical tissue during a first time period. A second signal is received from a second imaging ultrasound wave which has been reflected from the location in the anatomical tissue during a later second time period, following a discrete medical treatment. The second signal is subtracted from the first signal to form a difference signal. The difference signal may be scaled, spatially filtered, then used to generate an indication, the indication showing the effect of the medical treatment in the location in the anatomical tissue.

[0001] This is a continuation-in-part of application Ser. No.10/153,241, filed May 22, 2002, which claims priority to provisionalapplication serial No. 60/294,135 filed May 29, 2001. The presentinvention relates generally to ultrasound, and more particularly, to anultrasound medical imaging method.

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION

[0002] Ultrasound medical systems and methods include ultrasound imagingof anatomical tissue to identify tissue for medical treatment.Ultrasound may also be used to medically treat and destroy unwantedtissue by heating the tissue. Imaging is done using low-intensityultrasound waves, while medical treatment is performed withhigh-intensity ultrasound waves. High-intensity ultrasound waves, whenfocused at a focal zone a distance away from the ultrasound source, willsubstantially medically affect tissue in the focal zone. However, thehigh-intensity ultrasound will not substantially affect patient tissueoutside the focal zone, such as tissue located between the ultrasoundsource and the focal zone. Other treatment regimes of interest includeunfocused high-intensity ultrasound, wherein the ultrasound energy isdistributed over a relatively broad region of tissue rather than beinggenerally concentrated within a focal zone.

[0003] Ultrasound waves may be emitted and received by a transducerassembly. The transducer assembly may include a single element, or anarray of elements acting together, to image the anatomical tissue and toultrasonically ablate identified tissue. Transducer elements may employa concave shape or an acoustic lens to focus ultrasound energy.Transducer arrays may include planar, concave or convex elements tofocus or otherwise direct ultrasound energy. Further, such arrayelements may be electronically or mechanically controlled to steer andfocus the ultrasound waves emitted by the array to a focal zone toprovide three-dimensional medical ultrasound treatment of anatomicaltissue. In some treatments the transducer is placed on the surface ofthe tissue for imaging and/or treatment of areas within the tissue. Inother treatments the transducer is surrounded with a balloon which isexpanded to contact the surface of the tissue by filling the balloonwith a fluid such as a saline solution to provide acoustic couplingbetween the transducer and the tissue.

[0004] Examples of ultrasound medical systems and methods include:deploying an end effector having an ultrasound transducer outside thebody to break up kidney stones inside the body; endoscopically insertingan end effector having an ultrasound transducer into the rectum tomedically destroy prostate cancer; laparoscopically inserting an endeffector having an ultrasound transducer into the abdominal cavity todestroy a cancerous liver tumor; intravenously inserting a catheter endeffector having an ultrasound transducer into a vein in the arm andmoving the catheter to the heart to medically destroy diseased hearttissue; and interstitially inserting a needle end effector having anultrasound transducer into the tongue to medically destroy tissue toreduce tongue volume as a treatment for snoring. Methods for guiding anend effector to the target tissue include x-rays, Magnetic ResonanceImages (“MRI”) and images produced using the ultrasound transduceritself.

[0005] Low-intensity ultrasound energy may be applied to unexposedanatomical tissue for the purpose of examining the tissue. Ultrasoundpulses are emitted, and returning echoes are measured to determine thecharacteristics of the unexposed tissue. Variations in tissue structureand tissue boundaries have varying acoustic impedances, resulting invariations in the strength of ultrasound echoes. A common ultrasoundimaging technique is known in the art as “B-Mode” wherein either asingle ultrasound transducer is articulated or an array of ultrasoundtransducers is moved or electronically scanned to generate atwo-dimensional image of an area of tissue. The generated image iscomprised of a plurality of pixels, each pixel corresponding to aportion of the tissue area being examined. The varying strength of theechoes is preferably translated to a proportional pixel brightness. Acathode ray tube or liquid crystal display can be used to display atwo-dimensional pixellated image of the tissue area being examinedvarying strength of the echoes is preferably translated to aproportional pixel brightness. A cathode ray tube or liquid crystaldisplay can be used to display a two-dimensional pixellated image of thetissue area being examined.

[0006] When high-intensity ultrasound energy is applied to anatomicaltissue, significant beneficial physiological effects may be produced inthe tissue. For example, undesired anatomical tissue may be ablated byheating the tissue with high-intensity ultrasound energy. By focusingthe ultrasound energy at one or more specific focusing zones within thetissue, thermal effects can be confined to a defined region that may beremote from the ultrasound transducer. The use of high-intensity focusedultrasound to ablate tissue presents many advantages, including: reducedpatient trauma and pain; potentially reduced patient recovery time;elimination of the need for some surgical incisions and stitches;reduced or obviated need for general anesthesia; reduced exposure ofnon-targeted internal tissue; reduced risk of infection and othercomplications; avoidance of damage to non-targeted tissue; avoidance ofharmful cumulative effects from the ultrasound energy on the surroundingnon-target tissue; reduced treatment costs; minimal blood loss; and theability for ultrasound treatments to be performed at non-hospital sitesand/or on an out-patient basis.

[0007] Ultrasound treatment of anatomical tissue may involve thealternating use of both low-intensity imaging ultrasound andhigh-intensity treatment ultrasound. During such treatment, imaging isfirst performed to identify and locate the tissue to be treated. Theidentified tissue is then medically treated with high-intensityultrasound energy for the purpose of ablating the tissue. After a periodof exposure to high-intensity ultrasound, a subsequent image of thetissue is generated using low-intensity ultrasound energy to determinethe results of the ultrasound treatment and provide visual guidance tothe user to aid in subsequent treatments. This process of applyinglow-energy ultrasound to assist in guiding the position and focal pointof the transducer, followed by high-energy ultrasound to ablate theundesired anatomical tissue, may continue until the undesired tissue hasbeen completely ablated.

[0008] Although this conventional B-Mode ultrasound imaging provides aneffective means for imaging tissue that is in a static state, imaging ofthe tissue becomes more problematic when used in conjunction withthermal high-intensity ultrasound treatment. As the tissue is ablatedduring treatment, the heating effects of ultrasound upon the tissueoften result in qualitative changes in echo strength, causing brightnessvariations in the pixel display that do not consistently correspondspatially to the tissue being treated. These brightness variationsresult in an image display that does not represent the actual shape andsize of the region of tissue that is being thermally modified by thetreatment, introducing some visual ambiguity to the image.

[0009] Several methods are known for monitoring thermal ablation usingB-Mode ultrasound imaging. Most of these are based on changes in theenergy of ultrasound echoes, and include simple B-Mode displays of echoamplitude, estimates of tissue attenuation from analysis of distalshadowing, and quantification of changes in echo energy. Each of thesemethods have significant shortcomings because the tissue being treatedcan appear hyperechoic for reasons other than thermal ablation andbecause image changes must be qualitatively perceived by the user.

[0010] The most successful known methods for monitoring thermal ablationusing ultrasound are based on analysis of changes in echo energy ratherthan a direct analysis of the echo energy. Automatic and quantitativedisplays of changes in echo energy or tissue attenuation are possibleand can help users isolate thermally-induced changes from pre-existingecho characteristics. However, since such methods require integration ofechoes over substantial regions of an image scan or “frame,” theresulting images are very limited in spatial resolution. Although energyincreases (and therefore B-Mode brightness increases) correspond roughlyto lesion (i.e., the thermally treated tissue) position, typically theshape and size of the mapped energy increases do not always spatiallycorrespond to actual lesions, and sometimes are either absent orotherwise unapparent.

[0011] There is a need for an improved method of ultrasound imaging thatcan be utilized in conjunction with therapeutic ultrasound treatmentthat monitors the thermal effects of the treatment on targeted tissuewith greater accuracy and resolution.

SUMMARY OF THE INVENTION

[0012] The present invention overcomes the limitations of the known artby mapping differences between a first and second echo signal, eachsignal being obtained at different instants of time. The first andsecond signals are typically separated by a small time interval. Thefirst and second signals are processed, then a measure of the amplitudeof the differences between the first and second signals is made (ascontrasted with a measure of the differences in signal amplitude). Thisdifference signal is then spatially filtered and scaled to quantify echochanges associated with changes in tissue state. Difference signals maybe summed over multiple time periods to obtain a cumulative map of thechanges in the tissue. The resulting signals may be used to generate anultrasound image that is more representative of the tissue as treatmentprogresses, providing additional information about where thermal effectsare occurring. This allows for verification of successful treatment andmodification of unsuccessful treatment. Known ultrasound imaging andtreatment transducers may be used, providing users with increasedaccuracy without a need for special end effectors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Further features of the present invention will become apparent tothose skilled in the art to which the present invention relates fromreading the following specification with reference to the accompanyingdrawings, in which:

[0014]FIG. 1 is a flow diagram providing an overview of an ultrasoundtreatment method according to an embodiment of the present invention;

[0015]FIG. 2 illustrates the relative amplitude and timing of ultrasoundimage frames and ultrasound treatments of the method of FIG. 1;

[0016]FIG. 3 is a flow diagram of a method for monitoring medicaltreatment of anatomical tissue using pulse-echo ultrasound according toan embodiment of the present invention;

[0017]FIG. 4 is a view of a first ultrasound signal on a time scale;

[0018]FIG. 5 is a view of a second ultrasound signal on a time scale;

[0019]FIG. 6 is a composite view of the signals of FIG. 4 and FIG. 5;

[0020]FIG. 7 is a view showing the difference signal computed from FIG.4 and FIG. 5;

[0021]FIG. 8 is a view showing the absolute value of the differencesignal of FIG. 7;

[0022]FIG. 9 is a view of the signal of FIG. 8 after filtering;

[0023]FIG. 10 is a flow diagram depicting a method for monitoringmedical treatment of anatomical tissue using pulse-echo ultrasoundaccording to an alternate embodiment of the present invention;

[0024]FIG. 11 illustrates the relative amplitude and timing ofultrasound image frames, image frame sets and ultrasound treatments ofthe method of FIG. 10;

[0025]FIG. 12 depicts a flow diagram of a method for monitoring medicaltreatment of anatomical tissue using pulse-echo ultrasound according toanother alternate embodiment of the present invention;

[0026]FIG. 13 shows the relative amplitude and timing of ultrasoundimage frame sets, difference signals and ultrasound treatments of themethod of FIG. 12;

[0027]FIG. 14 is a flow diagram of a method for monitoring medicaltreatment of anatomical tissue using pulse-echo ultrasound according toyet another alternate embodiment of the present invention; and

[0028]FIG. 15 shows the relative amplitude and timing of ultrasoundimage frames and ultrasound treatments of the method of FIG. 14.

DETAILED DESCRIPTION

[0029] An overview of an ultrasound treatment method 10 according to anembodiment of the present invention is shown in FIG. 1. The methodbegins at step 12 by positioning proximate the targeted anatomicaltissue to be medically treated a transducer capable of transmitting andreceiving ultrasound signals. Once the transducer is in position,treatment begins at step 14 by emitting a high-intensity ultrasoundsignal from the transducer. The high-intensity ultrasound signalmedically treats the targeted tissue, such as (but not limited to)heating the tissue to ablate the material. At step 16 a low-intensityultrasound signal, such as a B-Mode signal, is emitted from thetransducer and the reflected signals are received to form a first imageframe F₁. It is understood that the terminology “image” includes,without limitation, creating an image in a visual form and displayed,for example, on a monitor, screen or display, and creating an image inelectronic form which, for example, can be used by a computer withoutfirst being displayed in visual form. After the first image frame F₁ isreceived at step 16, a predetermined waiting period is executed at step18 before proceeding further. It is to be understood that thepredetermined waiting period may vary in value from zero secondsupwardly to a maximum of several seconds, but preferably is in the rangeof milliseconds. After the predetermined wait period has been completed,a low intensity ultrasound signal is again emitted from the transducerand a second image frame F₂ is received at step 20. At step 22 adifference signal is derived from the image frames F₁ and F₂, as will bediscussed in greater detail below. The difference signal of step 22 isdisplayed as an image at step 24 to obtain a visual indication of thetissue change as a result of the medical treatment of step 14. It shouldbe noted that the visual indication of the tissue change provided by thepresent invention differs from the post-treatment image of the prior artin that the present invention provides an image showing echo differencesin contrast to echos from the target tissue. The image of echodifferences can indicate whether treatment is complete. If treatment iscomplete at step 26 (for example, the targeted tissue has been fullyablated), the method is ended at step 28. However, if the tissuerequires additional treatment, the transducer may be re-positioned atstep 30. The method then returns to step 14 to continue medicaltreatment of the targeted tissue.

[0030] Referring additionally to FIG. 2, the method of FIG. 1 isillustrated in relation to a time scale t. The targeted tissue ismedically treated with a relatively high-intensity ultrasound signal asat step 14. Then, at step 16 a relatively low-intensity B-Mode imagescan frame F₁ is received. After a predetermined off-time, as at step18, a second image frame F2 is received, as at step 20. The image framesF₁ and F₂ each contain a signal representing the same portion of thetarget tissue. For each image frame, a number of A-lines of raw echosignal data are received, the number of each line corresponding toazimuthal position and the signal time corresponding to depth.

[0031] Referring now to FIG. 3 in combination with FIGS. 1 and 2, amethod for monitoring medical treatment of anatomical tissue including,but not limited to, thermal ablation according to an embodiment of thepresent invention is depicted. An ultrasound transducer is positionedproximate the targeted anatomical tissue. The tissue may then bemedically treated such as by ablation using high-intensity ultrasoundwaves for a period of time as at step 14. At step 16 a first image frameF₁ (such as is illustrated in FIG. 4) is received. The image frame mayoptionally be stored electronically, such as in a computer, magneticmedia and solid-state memory. A second image frame F₂, separated from F₁by a fixed time interval of step 18, is received at step 20. An exampleimage frame F₂ is illustrated in FIG. 5. A difference signal is thenderived at step 22 by means of steps 32-38. The raw echo signals offrames F₁ and F₂ may be processed at step 32, such as to obtain complexanalytic signals by means of a Hilbert transformation. A differencesignal may then be derived by subtracting the signal of image frame F₂from the signal of F₁ at step 34. The difference signal of step 34 maytake into account both phase and amplitude differences between F₁ andF₂, computing the amplitude of the signal differences (as opposed todifferences in signal amplitude) of F₁ and F₂. A composite illustrationof image frames F₁ and F₂ is shown in FIG. 6, while the deriveddifference signal is depicted in FIG. 7. At step 36 the differencesignal may be scaled to a convenient value, such as the mean squaredvalue of the difference signal, the mean squared value of one of theoriginal echo signals, or a mathematical constant. As an example, asignal representing the scaled absolute value of the difference signalof FIG. 7 is shown in FIG. 8. Other functions of the difference signal,such as its squared absolute value or logarithm, may similarly beemployed. Still other scaling algorithms may use the amplitude and/orphase of the first and second signals to enhance differences between thefirst and second signals. Details of such algorithms are left to theskilled artisan. At step 38 the difference signal is spatially filtered,as depicted in FIG. 9, to smooth small-scale random variations. Spatialfiltering of the scaled difference signal is represented by Equation 1.Ψ(x, z) = ∫_(−∞)^(∞)∫_(−∞)^(∞)w(x − x, z − z₀)p₀(x₀, z₀) − p₁(x₀, z₀)²  x₀  z₀

[0032] Equation 1

[0033] In Equation 1 Ψ is a spatial difference map (image) of the scaledand filtered difference signal. The filtering may be performed byconvolution of the scaled difference signal with a two-dimensionalwindow w. This convolution may be efficiently performed through the useof two-dimensional Fast Fourier Transform (“FFT”) operations.

[0034] The difference signal may be normalized to have a maximum valueof 1. This approach would result in a spatial map of the echodecorrelation, similar to measures of turbulence in color Dopplerimaging systems. However, instead of examining echo decorrelation (anormalized measure of echo differences), a non-normalized map isconsidered preferable for the present invention because the echodifference is then enhanced in regions of greater echogenicity. Sincehyperechoicity is one correlate to tissue ablation, this featureincreases the specificity of the method for monitoring thermal ablativemedical treatment by providing an image with greater detail.

[0035] The spatially filtered signal of FIG. 9 is then displayed as animage at step 24 (see FIG. 3), in any manner previously discussed.

[0036] In a second embodiment of the present invention, ultrasoundimages may be generated as depicted in FIGS. 10 and 11. At step 40 thetissue is medically treated with high-intensity ultrasound waves. Atstep 42 a succession of image frames, depicted as F₁, F₂, . . . F_(n),are received. The image frames F₁, F₂, . . . F_(n) each contain a signalrepresenting the same portion of the target tissue. At step 44 the imageframes F₁, F₂, . . . F_(n) are mathematically grouped, such as bysumming or averaging, to form a first image frame set labeled FS₁, asshown in FIG. 11. After waiting a predetermined amount of time, as atstep 46, a second set of image frames F₁, F₂, . . . F_(n) are receivedat step 48. At step 50 the second set of image frames F₁, F₂, . . .F_(n) are mathematically grouped, such as by summing or averaging, toform a second image frame set FS₂ as shown in FIG. 11. The raw echosignals of image frame sets FS₁ and FS₂ may be processed at step 52,such as to derive complex analytic signals by means of a Hilberttransformation. The signal of image frame set FS₂ is then subtractedfrom the signal of image frame set FS₁ at step 54 to derive a differencesignal. The difference signal may take into account both phase andamplitude differences between FS₁ and FS₂, computing the amplitude ofthe signal differences (as opposed to differences in signal amplitude)of FS₁ and FS₂. At step 56 the difference signal may be scaled to aconvenient value using any scaling methods and algorithms, previouslydescribed or otherwise. At step 58 the difference signal is spatiallyfiltered to smooth small-scale random variations before being displayedas an image at step 59. This embodiment of the present invention mayprovide a more robust map of the backscatter changes by reducing theinfluence of random signal variations caused by rapid transient effectssuch as violent bubble activity produced during tissue ablation.

[0037] In a third embodiment of the present invention, smoothing of theimage signal may alternatively be accomplished by using a plurality ofimage frames, as illustrated in FIGS. 12 and 13. The tissue is medicallytreated at step 60, then a set of image frames F₁, F₂, . . . F_(n) arereceived at step 62. A plurality of difference signals D₁, D₂, . . .D_(n) are computed at step 64. It should be noted that the differencesignals may be computed using various arrangements of pairs of imageframes. For example, difference signal D₁ may be formed by subtractingF₂ from F₁; likewise, D₂ may be formed by subtracting F₃ from F₂, asshown in FIG. 13. Other arrangements of image frame pairs may also beused, including (but not limited to) odd-numbered image frames (i.e.,subtracting F₃ from F₁, etc.) and even-numbered image frames (i.e.,subtracting F₄ from F₂, etc.). The pairings may be interlaced (i.e.,subtracting F₂ from F₁, subtracting F₃ from F₂, etc.) or sequential(i.e., subtracting F₂ from F₁, F₄ from F₃, etc.). An indication or imagemay be displayed at step 66, showing at least one of the differencesignals D₁, D₂, . . . D_(n). At step 68 the difference signals D₁, D₂, .. . D_(n) may be further processed, such as by averaging, to reduceartifactual content. The averaged signal, denoted as {overscore (D)},may also be displayed as an image, as at step 70. The averaged signalsmay themselves be cumulatively summed, as at step 72, to provide a viewof the results of successive medical treatments 60. The summed averagesmay be displayed at step 74. If treatment is determined to be completeat step 76, the method is ended at step 78. However, if the tissueappears to require additional treatment, the transducer may bere-positioned at step 80. The method is then repeated beginning at step60 to continue treatment of the targeted tissue.

[0038] A fourth embodiment of the present invention is shown in FIGS. 14and 15 wherein a difference signal is derived using imaging framesgenerated both before and after medical treatment. At step 82 theultrasound transducer is positioned proximate the targeted tissue to bemedically treated. At step 84 a pre-treatment image frame F₁ isgenerated from received ultrasound signals. Then, at step 86 the tissueis subject to a quantum of medical treatment, such as by ablating thetissue. After a quantum of medical treatment is administered, a secondimage frame F₂ is generated at step 88. A difference signal is derivedat step 90, using the data contained in image frames F₁ and F₂ in thesame manner as previously described. An indication or image of thedifference signal may be displayed at step 92. If treatment isdetermined to be complete at step 94, the method is ended at step 96.However, if the targeted tissue is determined to require additionaltreatment, the transducer may be re-positioned as necessary at step 98.The method is then repeated and a subsequent quantum of treatment isadministered beginning at step 84.

[0039] An expected difficulty for the present invention is artifactualbackscatter change due to tissue motion artifacts. This difficulty canbe largely overcome by several features of the method. First,backscatter differences can be computed between image frames closelyspaced in time. If the tissue moves only a small amount during theinterval, motion artifacts are then small. Second, artifacts due toaxial tissue motion can be removed effectively by phase compensationduring signal processing. That is, before computation of the signaldifference, one of the complex image frames is multiplied by a phasecompensation function e^(−iθ), where θ is the low-pass filtered phase ofthe conjugate product of the two complex image frames. The resultingsignal difference is then computed, for example, using Equation 2:Ψ = ∫_(−∞)^(∞)∫_(−∞)^(∞)w(x, z)p₀(x, z) − p₁(x, z)^(−  ϖ₀δ  t)²  x  z

[0040] Equation 2

[0041] which is an improved echo difference map with reduced tissuemotion artifacts.

[0042] It is understood that one or more of the previously-describedembodiments, expressions of embodiments, examples, methods, etc. can becombined with any one or more of the other previously-describedembodiments, expressions of embodiments, examples, methods, etc. Forexample, and without limitation, any of the ultrasound transducers maybe used with other methods of medical treatment, such as producingimages to aid in tissue ablation by means of Radio Frequency (RF) andlaser energy, various non-ablative ultrasound medical treatments, andvarious ultrasound imaging applications.

[0043] The foregoing description of several expressions of embodimentsand methods of the invention has been presented for purposes ofillustration. It is not intended to be exhaustive or to limit thepresent invention to the precise forms and procedures disclosed, andobviously many modifications and variations are possible in light of theabove teaching. It is intended that the scope the invention be definedby the claims appended hereto.

What is claimed is:
 1. A method for ultrasound imaging of anatomicaltissue, comprising the steps of: a) receiving a first signal of a firstimaging ultrasound wave which has been reflected from a location in theanatomical tissue during a first time period; b) receiving a secondsignal of a second imaging ultrasound wave which has been reflected fromthe location in the anatomical tissue at a later second time periodfollowing a discrete medical treatment; c) subtracting the second signalfrom the first signal to derive a difference signal; d) generating anindication from the difference signal, the indication showing the effectof the discrete medical treatment in the location in the anatomicaltissue.
 2. The method of claim 1 wherein the first and second signalsare received after the discrete medical treatment has been completed. 3.The method of claim 1 wherein the first signal is received before thediscrete medical treatment, and the second signal is received after thediscrete medical treatment has been completed.
 4. The method of claim 1,further comprising the step of processing the first and second signals.5. The method of claim 4, further comprising the step of multiplying atleast one of the first and second signals by a phase compensationfunction to reduce motion artifacts.
 6. The method of claim 1, furthercomprising the step of scaling the difference signal.
 7. The method ofclaim 6 wherein the difference signal is scaled by squaring theamplitude of the difference signal.
 8. The method of claim 1, furthercomprising the step of spatially filtering the difference signal.
 9. Themethod of claim 1, wherein the medical treatment is ultrasound medicaltreatment.
 10. The method of claim 1, also including the steps a)through d) for different locations to image the anatomical tissue,wherein the image includes medically-treated locations andmedically-untreated locations of the anatomical tissue.
 11. The methodof claim 1, further comprising the step of combining the differencesignal image with a second image of the location in the anatomicaltissue.
 12. The method of claim 11 wherein the second image is generatedusing a B-Mode ultrasound scan.
 13. A method for ultrasound imaging ofanatomical tissue, comprising the steps of: a) receiving a first signalof a first imaging ultrasound wave which has been reflected from alocation in the anatomical tissue during a first time period; b)receiving a second signal of a second imaging ultrasound wave which hasbeen reflected from the location in the anatomical tissue at a latersecond time period following a discrete medical treatment; c) processingthe first and second signals; d) subtracting the second signal from thefirst signal to derive a difference signal; e) scaling the differencesignal; f) spatially filtering the difference signal; and g) generatingan indication from the difference signal, the indication showing theeffect of the discrete medical treatment in the location in theanatomical tissue.
 14. The method of claim 13 wherein the first andsecond signals are received after the discrete medical treatment hasbeen completed.
 15. The method of claim 13 wherein the first signal isreceived before the discrete medical treatment and the second signal isreceived after the discrete medical treatment.
 16. A method forultrasound imaging of anatomical tissue, comprising the steps of: a)receiving a first set of image frames comprising a plurality of imagingultrasound wave signals which have been reflected from a location in theanatomical tissue during a first period of time; b) receiving a secondset of image frames comprising a plurality of imaging ultrasound wavesignals which have been reflected from the location in the anatomicaltissue during a later second period of time following a discrete medicaltreatment; c) subtracting the imaging ultrasound signals of the secondset of image frames from the imaging ultrasound signals of the first setof image frames to derive a difference signal; and d) generating anindication from the difference signal, the indication showing the effectof the discrete medical treatment in the location in the anatomicaltissue.
 17. The method of claim 16 wherein the first and second sets ofimage frames are received after the discrete medical treatment has beencompleted.
 18. The method of claim 16 wherein the first set of imageframes is received before the discrete medical treatment, and the secondset of image frames is received after the discrete medical treatment.19. The method of claim 16, further comprising the step of processingthe first and second sets of signals.
 20. The method of claim 16,further comprising the step of scaling the difference signal.
 21. Themethod of claim 20 wherein the difference signal is scaled by squaringthe amplitude of the difference signal.
 22. The method of claim 16,further comprising the step of spatially filtering the differencesignal.
 23. The method of claim 16, wherein the medical treatment isultrasound medical treatment.
 24. The method of claim 16, also includingthe steps a) through d) for different locations to image the anatomicaltissue, wherein the image includes medically-treated locations andmedically-untreated locations of the anatomical tissue.
 25. A method forultrasound imaging of anatomical tissue, comprising the steps of: a)receiving a first set of image frames comprising a plurality of imagingultrasound wave signals which have been reflected from a location in theanatomical tissue during a first period of time; b) receiving a secondset of image frames comprising a plurality of imaging ultrasound wavesignals which have been reflected from the location in the anatomicaltissue during a later second period of time following a discrete medicaltreatment; c) processing the first and second sets of signals; d)subtracting the imaging ultrasound signals of the second set of imageframes from the ultrasound signals of the first set of image frames toderive a difference signal; e) scaling the difference signal; f)spatially filtering the difference signal; and g) generating anindication from the difference signal, the indication showing the effectof the discrete medical treatment in the location in the anatomicaltissue.
 26. The method of claim 25 wherein the first and second sets ofimage frames are received after the discrete medical treatment has beencompleted.
 27. The method of claim 25 wherein the first set of imageframes is received before the discrete medical treatment, and the secondset of image frames is received after the discrete medical treatment.28. The method of claim 25 wherein the medical treatment is ultrasoundmedical treatment.
 29. The method of claim 25, also including the stepsa) through g) for different locations to image the anatomical tissue,wherein the image includes medically-treated locations andmedically-untreated locations of the anatomical tissue.
 30. A method forultrasound imaging of anatomical tissue, comprising the steps of: a)providing a discrete medical treatment to the anatomical tissue; b)receiving a set of image frames comprising a plurality of imagingultrasound wave signals which have been reflected from a location in theanatomical tissue; c) pairing together a plurality of image frames, eachpair comprising a first image frame and a second image frame such thatthe second image frame has been reflected from a location in theanatomical tissue at a later time than the first image frame; d)subtracting the signals of the second image frame from the signals ofthe first image frame, for each pair of image frames in the image frameset, to derive a set of difference signals; e) using at least onedifference signal to generate an indication showing the effect of thediscrete medical treatment in the location in the anatomical tissue; andf) repeating steps a) through e) until medical treatment is completed.31. The method of claim 30, further comprising the steps of: a)computing an average of the set of difference signals; and b) using theaverage of the set of difference signals to generate an indicationshowing the effect of the discrete medical treatment in the location inthe anatomical tissue.
 32. The method of claim 31, further comprisingthe steps of: a) cumulatively summing the averages of the set ofdifference signals; and b) using the cumulative sum of averages of theset of difference signals to generate an indication showing the effectof the discrete medical treatment in the location in the anatomicaltissue.